Ultraviolet-reflecting eggs (controls) were more frequently retrieved to the nest-cup than non-reflecting (–UV) eggs. The experimental approach consisted of the manipulation of UV reflectance of the experimental eggs introduced outside the nest-cup of the hole-nester spotless starling Sturnus unicolor and the study of the retrieval of these eggs. Analyses based on raw data and controlling for phylogenetic distances both at the species and the family levels revealed that hole-nester species produced eggs with higher UV reflectance than those nesting in open habitats. First, we studied variation in egg coloration of 98 species of European passerines measured using UV–visible reflectance spectrometry (300–700 nm) in relation to nesting habits. Here, we tested this hypothesis by using comparative and experimental approaches. Owing to the conspicuousness of ultraviolet (UV) colour in dark environments, natural selection might have selected UV egg coloration because it would enhance egg detectability by parents in murky nests.

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